What TJ said.
Aviemore would be a good base. Could tick off some trail centres en-route. GT or Drumlanrig in the borders, Laggan further north. Then you got plenty of great lower level (altitude not ability!) riding around Aviemore if the weather aint great, but still have the option of doing something a bit more wild and remote if you like.
Also gives you the option of stopping off at other good spots along the A9 if you got the time (e.g. Dunkeld & Pitlochry)